Roland Juno 106 (#8) – common problems with the Juno chorus

Roland Juno 106
yet another Juno 106

Since I always do pretty much the same work on every Juno and have posted about it 7 times, this time I’m going to go more into depth about two specific, very common problems a Juno 106 chorus/jack board can have.

1. Symptom: you only hear one channel of the stereo chorus in the mono mix (“Mono/Left Output”), but it sounds normal through headphones. Your left output will sound like instead of a proper chorus you just have a yucky sounding, sort of steppy pitch modulation.

This often happens just because of cold solder joints on the solder tabs of the switching jacks for left and right output. These jacks are supposed to send the right channel of the chorus in to mix with the left channel right on the left jack when there is no cable plugged into “Right Output.” All you have to do is reflow the solder on the tabs of those jacks.

2. Symptom: you hear distortion when chorus is turned on and your VCA/filter output is reaching a high level (VCA gain is set high and/or envelope is at a high point). It will sound like overdrive or “analog clipping.” This means your chorus circuit is being overdriven because its bias is incorrect. The solution is to adjust the chorus bias trimmers on the jack board according the procedure in the service manual (Step 11 in the Adjustment procedure).

If you don’t have a signal generator and oscilloscope though, it is possible to do it by ear. First, plug a cable into each output jack (left and right) but only plug the left one into an amp.  Make sure you have an oscillator waveform selected, turn VCF Env all the way up and VCA Gain all the way up and turn on a chorus. Play a chord and adjust trimmer 1 on the jack board to the point where you hear the least distortion at the high point of the envelope (hopefully none).

Repeat the procedure with only the right output plugged into the amp, turning trimmer 2 on the jack board until the high point of the envelope is not distorted.

10 thoughts on “Roland Juno 106 (#8) – common problems with the Juno chorus”

  1. Fantastic blog Alison, I read your synth repair comments with lots of attention. I’ve been repairing my own gear for about a year and information like this is invaluable.

    I’m just about to look at a JX-3P with chorus going on on the right, so thanks for the jack board suggestion. Something tells me the bypass transistor on the chorus circuit for that channel is gone, however. I’ll soon find out.

  2. My stereo channel is flaky. I need to apply pressure to the level switch for it to work. Do you think this is cold solder joints or is it normal for the level switch to go bad?

    1. Hi Dan, it’s actually pretty common for the output level switch to be so dirty that it seriously affects the quality of the output! I’ve seen Juno 106s in which just a dirty output level switch made the output so quiet and crackly the synth was unusable. The solution is simply to clean it. Spray some electrical contact cleaner (withOUT lubricant) right into the switch from the outside and switch it back and forth a couple dozen times. It’s likely this will solve your output problem, but if it doesn’t, a reflow of the solder on the output jacks would be my next step.

  3. Hi there!
    Today i noticed that chorus 1 and chorus 2 on my Juno 60 have no difference between them. Wtf??? I’m pretty sure chorus had a huge difference from chorus 1. Now if you listen to the synth without playing anything you can notice there is a difference between chorus 1 and 2. The phasing is faster on chorus 2, but when playing music lets say an arpeggio and you engage chorus 1 and then 2 you notice no difference. Am i going crazy? I’m pretty sure there was a HUGE difference.

  4. Great work here. You’ve helped me so much.

    I have the distorted Chorus issue when playing chords, both waveforms on, and VCA is high. I did make the bias adjustment with an oscilloscope and used a euro modular oscillator to produce a sinewave. I could only get about 8.5V p to p with it, so I boosted it a tad with another module to reach 10V. I then attached alligator clips to the audio cable from the modular to the board test points.

    Manual says have VCA at 0, there wasn’t enough signal to see the wave folding, unfortunately. I had to raise the VCA up to see the clipping. I then made my adjustments. Unfortunately, I still have the clipping/overdriven chorus issue.

    Any other thoughts on the issue?

    1. The next step is to replace the MN3009 bucket brigade delay line ICs with the new Xvive clones that you can now get on ebay from Cabin Tech. We’ve noticed that as the original MN3009s wear out and age, their threshold for clipping lowers and we get a lot more distortion from them. When I made this post a couple years ago, the MN3009 BBD ICs were not in production, so replacing them wasn’t an option, but we’ve been replacing them very often since they came back on the market, both to get rid of the oceany background noise and reduce distortion from clipping.

      1. So I have the chips and I’m in the midst of replacing them. I noticed a resistor and jumper wire has been soldered to IC10 pin3 and R114. Does this need to be connected back onto these new chips? Not sure if this is a mod/repair, or how it came originally.

    2. I had this issue in one of my 106 repairs and it turned out to be faulty muting transistors. I had to replace them with the exact same part to make them work consistently with the VCA and chorus levels.

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